Polymeric solids have many unique advantages over other materials and have therefore risen in importance in recent years. For example, polymers are lightweight, can be molded into intricate shapes, are corrosion resistant, have versatile electronic properties and low manufacturing costs. However, due to their inherently low melting temperatures and susceptibility to degradation in oxidizing and/or UV environments, their use has been generally limited to environmentally mild service applications. Carbon-based composite materials, for example, carbon fiber-reinforced plastic (CFRP) composites and carbon fiber-carbon composites, are light weight, tough and rigid, making them very attractive materials for spacecraft components. However, like polymers, composites are also eroded in extremely oxidative environments.
The use of polymers and composites in spacecraft applications exemplifies this problem. To date, the widespread use of polymers and composites in space, on a prolonged basis, has not been possible due to their low erosion resistance in the presence of oxygen, particularly the active oxygen species, such as atomic oxygen, that can be found in the residual atmosphere surrounding the Earth in low earth orbit (LEO).
The LEO space environment contains atomic oxygen formed in the ionosphere through dissociation of O.sub.2 by vacuum ultraviolet radiation (VUV) having a wavelength in the range of about 100 to 200 nm. The predominant species in the LEO environment, at altitudes between 200 and 700 km, is atomic oxygen (AO). Even at higher altitudes, AO remains a significant constituent.
AO density in LEO is not particularly high at the altitudes of most orbiting vehicles, such as satellites. For example, the number density of AO at about 250 km altitude is 10.sup.9 atoms/cm.sup.3, which corresponds to the density of residual gas in a vacuum of 10.sup.-7 torr. However, due to the high orbital velocity (approximately 8 km/sec at space shuttle altitude) of orbiting vehicles, the flux is high, being of the order of 10.sup.15 atoms/cm.sup.2 sec. Furthermore, this high orbital velocity gives the impacting oxygen considerable energy, about 5.3 eV. AO having kinetic energy above about 1 eV, and more typically in the range of from about 2 eV to about 5 eV, is commonly referred to as "fast atomic oxygen" (FAO) or "hyperthermal atomic oxygen" (HAO).
Polymeric materials, graphite and carbon-based composite materials exposed to such energetic FAO have been shown to undergo surface erosion and mass loss. Over time, this surface erosion can result in degradation and failure of these materials. In addition, eroded materials exhibit a significantly altered surface morphology, the surface being roughened, often producing a micron scale, "carpet-like " texture.
It has been found that erosion theoretically attributable to FAO alone does not adequately account for the observed rate of erosion of certain polymeric materials. It is believed that FAO and UV in the LEO environment act synergistically to accelerate degradation of these polymeric materials. Furthermore, although atomic oxygen and UV radiation cause the most damage to polymeric surfaces, polymers may also be damaged in space by thermal cycling and micrometeoroid/debris impact.
Many different types of polymers and composites have been examined in LEO flight and in ground based FAO testing facilities. Polymers which are commonly considered for use in the LEO environment include Kapton.TM. polyimide, FEP teflon (fluorinated ethylene propylene), PFE teflon (polytetrafluoroethylene), Mylar.TM. (Polyethylene terephthalate), and PEEK.TM. (poly ether ether ketone). Also used are composite materials such as carbon fiber-carbon composites comprising carbon fibers in a resin-derived carbon matrix, and CFRP composites such as carbon fibers bonded with epoxy resins or PEEK.
Kapton and epoxies have LEO erosion yields of about (2.5-3).times.10.sup.-24 cm.sup.3 /at, which translates to (3-4).times.10.sup.-24 g/atom of atomic oxygen. Many other polymers and carbon-based materials, such as graphite, carbon fiber-carbon composites and CFRP composites also have erosion rates of this order of magnitude, typically about (1-4).times.10.sup.-24 g/atom. Perfluorinated polymers are an exception, because of the fluorine in their bonding structure, their erosion yields are much lower. Although it was once thought that perfluorinated polymers were an answer to the problems of polymers in LEO, there is a synergistic effect between atomic oxygen and VUV radiation that increases the erosion yield to unacceptable levels. Materials having erosion yields on the order of 10.sup.-24 g/atom are unsuitable for long term use in the LEO environment, and space in general.
In CFRP or carbon fiber-carbon composites, the top 10-20 .mu.m of material usually consists of a polymeric or carbon matrix, respectively, with carbon or graphite fibers bonded in the matrix below the surface. In long-duration space missions, erosion of both the matrix and the carbon fibers has been observed. These materials have erosion rates on the order of 10.sup.-24 g/atom and are therefore unsuitable for long term use in the LEO environment without alteration or protection.
Some specialized polymeric materials have been developed having acceptable erosion resistance for use in short term space flights. However, the cost of developing new materials for use in space is very high. Therefore, it is preferred to use existing, industrially produced polymeric materials due to their lower cost, wide availability and well understood properties. In particular, it is preferred to surface-modify existing polymers to improve their erosion resistance while retaining the properties of the unmodified bulk polymer.
The advantages of using existing organic polymers has forced the development of a wide variety of protection schemes, ranging from simple blankets of glass cloth to sophisticated thin film coatings. It is known that these coatings are most often fashioned using silicon dioxide. The coatings comprise thin films deposited by chemical vapor deposition or electron or ion beam sputtering onto the polymer surface to act as a barrier between the polymer and atomic oxygen.
An example of such a thin film coating is disclosed in U.S. Pat. No. 5,424,131 (Wertheimer et al.). Wertheimer teaches the deposition of a thin barrier film, preferably via a plasma, on the surface of an organic polymer such as Kapton, Mylar or epoxy resin. The materials comprising the films are plasma polymers, inorganic insulating films, and semiconducting and conductive materials. As shown in the examples of Wertheimer, all of these materials preferably contain silicon. After the polymers were coated, they were exposed to oxygen atoms in a simulated LEO environment. Although not disclosed by Wertheimer, this exposure to oxygen atoms would convert the silicon-containing coating on the polymer surface into a coating of silicon dioxide.
In one example of Wertheimer, Kapton is coated with a thin film of the plasma polymer hexamethyldisiloxane and subsequently exposed to AO. A skin of SiO.sub.2 forms on top of the plasma polymerized HMDSO coating during the initial stages of AO exposure, thereby protecting it from further attack and fulfilling the protection from the AO exposure. However, the cyclic mode of degradation of siloxane protective coatings in an oxygen environment implies an induction period prior to the onset of measurable degradation. This may be the reason why the siloxane materials flown in short duration space shuttle flights have appeared to be stable over the time period during which they were exposed. For extended space flights, however, these coating materials have a finite lifetime determined by their thickness and the AO flux. This problem is widely recognized in the space community.
Therefore, although SiO.sub.2 coated materials such as those taught by Wertheimer have improved resistance to atomic oxygen, they are unsuitable for use in space on a long term basis.
It has also been found that when coated materials are exposed to constant thermal cycling, as in the LEO environment, cracking and spalling of the coating quickly occur leaving the underlying polymer exposed. This results in the erosion of the exposed polymer, erosion that is enhanced by undercutting of the coating, causing rapid widening and deepening of the cracked and eroded area. It is believed that the cracking and spalling in oxide coatings is primarily caused by the difference between the coefficient of thermal expansion of the coating and that of the underlying bulk material, and also due to interfacial stresses at the interface between the coating and the bulk material. High interfacial stresses in coated materials are caused by the typically sharp transition between the coating and the underlying bulk material. In order to provide full protection in the LEO environment, any new protection scheme must have resistance to thermal stress induced cracking and spalling.
It is known that selective silylation processes are used with positive polymer-based photoresists in the manufacture of semiconductor devices. The SiO.sub.2 type enriched layer of these photoresists is typically formed by silylation of active groups in exposed areas of the photoresist followed by plasma development of the resist by reactive oxygen plasma etching. The SiO.sub.2 type enriched layer is typically formed during the first few moments of etching. Silylated areas of the surface are etched at a lower rate than unsilylated areas, resulting in an etched pattern in the surface of the photoresist.
As shown in the prior art, the polymer comprising the photoresist must contain reactive hydrogen groups such as COOH (carboxyl), OOH, OH, NH and SH in order to react with the silylating agent. The silylating agent reacts with the reactive hydrogen group, replacing the hydrogen atom with a silyl group.
The requirement that the polymer contain reactive hydrogen groups limits the types of polymers which may be silylated. Typically this process has been used only to silylate polymers having phenolic hydroxyl groups, such as novolak resins, or those polymers having reactive precursor groups. Known precursor groups are o-nitrobenzene derivatives and other compounds which undergo photo-Fries rearrangement, or epoxides, which undergo ring opening by chemical means, to form reactive hydrogen groups. An example of a process for silylating polymers having reactive hydrogen groups or active hydrogen precursor groups is taught by Babich in U.S. Pat. No. 4,782,008.
It is also known that UV radiation can initiate oxidation of a polymer by molecular oxygen, and change the surface characteristics thereof. For example, in U.S. Pat. No. 5,098,618, Zelez teaches that UV irradiation of polymers in the presence of oxygen is believed to produce oxide and possibly hydroxide sites on polymer surfaces not previously containing oxygen. However, Zelez deals primarily with improving hydrophilicity of polymer surfaces, and no effort is made to quantify the relative amounts of reactive hydrogen groups and other oxygen-containing groups produced by irradiation. It is likely that the high energy of irradiation used by Zelez (about 10-15 mWatts/cm.sup.2) would result primarily in the production of ketone carbonyl groups and would produce relatively few hydroxyl or other reactive hydrogen groups.
Despite the fact that separate processes are known to: 1) perform silylation of polymers, ie. convert active hydrogen groups to silyl groups, and 2) introduce oxygen into a polymer surface that did not previously contain oxygen, no complete process has been developed that successfully applies these concepts to the production of polymer materials that, for example, have superior resistance to degradation in a highly oxidizing environment, such as in LEO.
Firstly, no processes are known that allow a wide range of organic polymers to be silylated. As discussed above, silylation has only been demonstrated for polymers having reactive hydrogen functional groups or reactive hydrogen functional precursor groups. If, again, spacecraft applications are used as an example, it can be said that none of the polymer materials traditionally used have been silylated. The simple reason for this is that these polymers have high thermal, chemical and mechanical stability and are therefore difficult to surface-modify chemically. The technique of silylation has not been applied, previous to this development, because these polymers do not contain reactive functional groups or reactive functional precursor groups which may be readily converted. In other words, these polymers are substantially unreactive with silylating agents.
Secondly, it has not been specifically demonstrated that a large number of reactive hydrogen groups can be produced on the surface of the polymers mentioned above. The production of the reactive hydrogen groups, which must be present in very specific forms and of very high quantity, has only been demonstrated in a cursory fashion in a non-related manner, as for example by the above-mentioned Zelez patent in the simple control of hydrophobicity.
Thirdly, to date it has not been possible to provide a silicon and oxygen enriched layer on the surface of a polymer which would not be subject to cracking and/or spalling in high thermal stress environments, such as the LEO environment.
As a second example, the requirements for barrier films in the packaging industry mirror, in many ways, the requirements of the aerospace industry. The film materials must be stable in oxidizing environments (food contact and heat), maintaining low permeation of water vapor and oxygen, for example. Once again the materials that are traditionally used, such as polyethylene and polyethylene terephthalate, do not contain reactive hydrogen groups or reactive precursor groups that can be converted, and therefore substantially unreactive with silylating agents. Like the aerospace polymers, barrier films have never been silylated or surface modified to form a silicon dioxide enriched surface layer, due to the difficulty of forming new chemical bonds with these materials. It is known that providing a thin film coating of silicon dioxide on packaging films by chemical vapor deposition will enhance the oxygen and water vapor barrier properties of the film. However, at the present time, such films are costly to produce and the deposited oxide film is subject to cracking and spalling under mechanical stress, for example when the material is handled roughly, crushed, bent or folded.